Try the following method. 1. Restart your PC 2. Completely delete the GameGuard folder. (Go to C: > Program Files > Webzen > Mu > GameGuard) 3. Start MU Online.
The error message popped up after the game crashed. It was just a tiny little notification window from GG that closed itself suspiciously quick on its own.
Your gameguard is probably already running, try closing it via task manager, it goes by the name nProtect Game Monitor if i'm not mistaken, do it whenever ...